1. Field of the Invention
The invention relates to a capacitive fingerprint sensor, and more particularly to a capacitive fingerprint sensor without analog-to-digital converters (ADCs).
2. Description of the Related Art
In recent years, biological identification technology has become increasingly mature, and different biological features can be used for identifying users. Since the recognition rate and accuracy of fingerprint identification technology are better than those of other biological feature identification technologies, fingerprint identification and verification are used extensively in various areas.
Fingerprint identification and verification technology detects a user's fingerprint image, captures fingerprint data from the fingerprint image, and saves the fingerprint data as a template. Thereafter, the user presses or slides the finger on or over the fingerprint sensor such that a fingerprint is captured and compared with a template. If the two match, then the user's identity is verified.
As portable devices become increasingly popular, using portable electronic devices to input fingerprint data is becoming a trend. Therefore, a fingerprint sensor with a smaller area is desired.
When a user presses or slides his finger on or over the fingerprint sensor, a conventional capacitive fingerprint sensor will sense the ridges and the valleys of the fingerprint, and generate different capacitance values corresponding to the ridges and valleys. Next, voltage values corresponding to the capacitance values are obtained by using a charge-sharing technique, and the voltage value is input to an analog-to-digital converter (ADC) for converting the voltage value into a digital code. The digital code is provided to a processor for subsequent operation and fingerprint identification.